<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>过渡练习</title>

    <style>
        * {
            padding: 0;
            margin: 0;
        }
        
        .box {
            width: 200px;
            height: 15px;
            border-radius: 30px;
            border: 1px solid turquoise;
        }
        
        .box_t {
            width: 25%;
            height: 100%;
            border-radius: 30px 0 0 30px;
            background-color: turquoise;
            transition: 1.5s;
        }
        
        .box_t:hover {
            border-radius: 30px;
            width: 100%;
        }
    </style>
</head>

<body>


    <div class="box">
        <div class="box_t">

        </div>

    </div>



</body>

</html>